Production of hydrocarbon fuels presently takes many forms. Often, oil wells that have been abandoned previously due to pressure loss are the subject of secondary or tertiary recovery. In some systems, a compressed gas is injected into the oil well to force liquid hydrocarbons into a recoverable position in subsurface formations and to provide reservoir pressure. It is desirable, when injecting gas within an oil bearing formation, to monitor the movement of gas with time. Ideally, this should be accomplished with minimal cost and shut-down time.
Methods that are in practice today include the evaluation of porosity computed from the compensated neutron and/or water saturation computed from a pulsed-neutron log. Neutrons have no electrical charge and have a mass very close to that of a proton. Since there is no ordinary electrostatic repulsion or attraction, a neutron will undergo collision only rarely. A direct hit on an electron or a nucleus must occur before a neutron is detected.
A neutron can interact with other particles in two ways, depending upon the energy of the neutron and the nucleus involved. In the first type, low energy neutrons (&lt;0.025 eV) are captured by a nucleus, increasing its internal energy and immediately releasing this energy in the form of gamma radiation. One can identify the capture nucleus by the pattern of its released gamma radiation. Pulsed-neutron logging utilizes this method.
The second type of neutron interaction is called scattering. There are two types of potential collisions which a neutron may have, inelastic and elastic. Inelastic scattering takes place when the neutron is highly excited (&gt;100 eV). This most commonly occurs early in the neutron's life. This type of scattering transfers both kinetic and internal energy from the excited neutron to the target nucleus.
Elastic scattering is the primary mechanism by which the neutron loses energy. The loss of energy from the neutron to the nucleus is in the form of kinetic energy. The neutron will continue to collide with various elements until it decelerates to its lowest energy level. Neutrons residing at this level are called thermal neutrons (&lt;0.025 eV). They are still colliding, but the net energy transfer is zero. Thermal neutrons meander by means of thermal diffusion and continue for a period of time which depends upon the material in which they are diffused. The size of this neutron population will depend upon porosity and hydrogen content. Hydrogen dominates the neutron deceleration process due to its mass relative to that of a neutron. It is this neutron population which is detected by the compensated neutron instrument.
A pulsed neutron log is provided by the use of a pulsed neutron tool. The neutron source for this instrument is an electronic neutron generator. This source emits packets of high energy neutrons into the formation. These neutrons are then decelerated, by the process discussed above, and captured, releasing gamma radiation. These captured gamma rays are the input data used for computing porosity and water saturation. An estimate of water salinity is necessary for the computation of water saturation.
Both of these methods are effective. However, each requires a neutron moderating fluid in the borehole. The expense of filling the borehole with fluid coupled with extracting the fluid after the logs have been run, and returning the well to production, is often exorbitant. Consequently, many gas-injector fields go unmonitored.